An antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are then radiated into space. The antenna may also convert electromagnetic waves into electric current. The electric field or “E” plane determines the polarization or orientation of the radio wave. In general, most antennas radiate either linear or circular polarization.
A linearly polarized antenna radiates in one plane. In a circularly polarized antenna, the plane of polarization rotates in a circle making one complete revolution during one period of the wave. An antenna is said to be vertically polarized (linear) when its electric field is perpendicular to the Earth's surface. An example of a vertical antenna is a broadcast tower for AM radio or the “whip” antenna on an automobile.
Horizontally polarized linear antennas, such as dipole turnstiles, wire loops and slotted cylinders, have their electric field parallel to the Earth's surface. Television transmissions in the United States typically use horizontal polarization.
Present day omnidirectional horizontally polarized antennas, such as turnstile dipoles, wire loops and slotted cylinders, have limited bandwidth. For example, U.S. Pat. No. 6,414,647 to Lee discloses a circularly polarized slot-dipole antenna, where the slot and the dipole are located in the same physical structure. The antenna includes two substantially cylindrical members with a slot located on the outer surface of the antenna.
Conical antennas, which include a single inverted cone over a ground plane, and biconical antennas, which include a pair of cones oriented with their apexes pointing toward each other, are used as broadband antennas for various applications, for example, spectrum surveillance. Conical antennas have previously been known as the wave transformer or omnidirectional horn antennas, and are noted for their great bandwidth.
Inventorship of the Biconical Dipole Antenna has been attributed to Sir Oliver Lodge in 1897. Wire cage conical monopole antennas were used by 1905, at the Marconi Transatlantic Stations. Later, a biconical dipole antenna including a coaxial feed structure, was disclosed in U.S. Pat. No. 2,175,252 to Carter entitled “Short Wave Antenna”.
Excitation of biconical dipoles is accomplished by imparting an electrical potential across the apex of the two opposing cones, causing a TEM mode. This mode is analogous to the TE01 mode of sectoral horns, but as the biconical dipole is a complete figure of revolution, symmetric about the cone axis, the TEM mode results. In a sectoral horn, a monopole probe is commonly used for excitation. In a biconical dipole, excitation is by the dipole moment formed across the horn walls (opposing cones), so the structure is self exciting. A biconical dipole antenna is an example of an omnidirectional vertically polarized antenna of relatively great bandwidth.
TE10 modeling of conventional biconical dipole structures has been proposed for the purpose of horizontal polarization and omnidirectional radiation. In one instance, a circle of wire operates as loop antenna and excitation probe, and is placed normal to the bicone axis (Chu et. al., “Biconical Electromagnetic Horns”, Proceedings of the IRE, Vol. 27, page 769, December 1939). In this approach, the cones act only as horn walls and they are not self exciting. Gain bandwidth of this system is limited, due to the narrow bandwidth of the wire loop probe.
Loop antennas relate to circles, and they can be open or closed, as in the hole of wire loop or the solid center of a metal disc antenna. Current can be conveyed in a circle, as around the rim of metal disc, the periphery of a hole in a metal sheet, or along a circular ring of wire. Solid planar loop antennas not having an open aperture, formed in or of a metal sheet, are slot antennas and operate according to Babinet's Principle. Slot antennas can be either loop or dipole, according to their shapes, as circles or lines.
Planar slot equivalent loops, such as the circular metal disc antenna, are less common than planar slot dipoles in the radio field, yet their use can be preferential. In one tradeoff, slot loops of one wavelength circumference operate at 2nd harmonic or antiresonance, to provide a nominal 50 ohm driving resistance. A preferred method of excitation is to notch the metal sheet from which they are formed, such as to cause a discontinuity. Slot loops are therefore easily implemented.
Antennas then, can be divided into two canonical forms including the dipole antenna and the loop antenna, which correspond to the capacitor and inductor of RF electronics, having radial near fields that are electric or magnetic respectively. Thus, radiation may be caused by two distinct mechanisms including separation of charge in dipoles and conveyance of charge in loops. The dipole relates to the line while the loop relates to the circle. While broadband dipoles are known in the art, for example, the biconical and bowtie dipoles, the broadband forms of loop antennas have largely been unknown.
The conical and spatial, or 3-D volumetric form, of dipoles is well known, being the biconical dipole antenna. The conical and spatial form of loop antennas has however, remained unknown. Consequently, there still is a need for a broadband omnidirectional horizontally polarized antenna, and a dual to compliment the biconical dipole.